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6 Systems Analysis and Design in a Changing World, Fifth Edition
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6 Learning Objectives Explain how the traditional approach and the object- oriented approach differ when modeling the details of a use case List the components of a traditional system and the symbols representing them on a data flow diagram Describe how data flow diagrams can show the system at various levels of abstraction Systems Analysis and Design in a Changing World, 5th Edition2
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6 Learning Objectives (continued) Develop data flow diagrams, data element definitions, data store definitions, and process descriptions Develop tables to show the distribution of processing and data access across system locations Systems Analysis and Design in a Changing World, 5th Edition3
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6 Overview What the system does and what event occurs – activities and interactions (use case) Traditional structured approach to representing activities and interactions Diagrams and other models of the traditional approach RMO customer support system example shows how each model is related Systems Analysis and Design in a Changing World, 5th Edition4
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6 Traditional vs. Object-Oriented Approaches Systems Analysis and Design in a Changing World, 5th Edition5 Figure 6-1
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6 Requirements for the Traditional and OO Approaches Systems Analysis and Design in a Changing World, 5th Edition6 Figure 6-2
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6 Data Flow Diagrams (DFDs) Graphical system model that shows all main requirements for an IS in one diagram Inputs/outputs Processes Data storage Easy to read and understand with minimal training Systems Analysis and Design in a Changing World, 5th Edition7
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6 Data Flow Diagram Symbols Systems Analysis and Design in a Changing World, 5th Edition8 Figure 6-3
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6 DFD Fragment Showing Use Case Look Up Item Availability from the RMO Systems Analysis and Design in a Changing World, 5th Edition9 Figure 6-4
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6 DFD Integrates Event Table and ERD Systems Analysis and Design in a Changing World, 5th Edition10 Figure 6-5
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6 DFD and Levels of Abstraction Data flow diagrams (DFDs) are decomposed into additional diagrams to provide multiple levels of detail Higher-level diagrams provide general views of system Lower-level diagrams provide detailed views of system Differing views are called levels of abstraction Systems Analysis and Design in a Changing World, 5th Edition11
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6 Layers of DFD Abstraction for Course Registration System Systems Analysis and Design in a Changing World, 5th Edition12 Figure 6-6
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6 Context Diagrams DFD that summarizes all processing activity for the system or subsystem Highest level (most abstract) view of system Shows system boundaries System scope is represented by a single process, external agents, and all data flows into and out of the system Systems Analysis and Design in a Changing World, 5th Edition13
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6 DFD Fragments Created for each use case in the event table Represent system response to one event within a single process symbol Self-contained models Focus attention on single part of system Show only data stores required in the use case Systems Analysis and Design in a Changing World, 5th Edition14
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6 Three Separate DFD Fragments for Course Registration System Systems Analysis and Design in a Changing World, 5th Edition15 Figure 6-7
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6 Event-Partitioned System Model DFD to model system requirements using single process for each use case/activity in system or subsystem Combines all DFD fragments together to show decomposition of the context-level diagram Sometimes called “diagram 0” Used primarily as a presentation tool Decomposed into more detailed DFD fragments Systems Analysis and Design in a Changing World, 5th Edition16
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6 Combining DFD Fragments to Create Event- Partitioned System Model Systems Analysis and Design in a Changing World, 5th Edition17 Figure 6-8
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6 Context Diagram for RMO Customer Support System Systems Analysis and Design in a Changing World, 5th Edition18 Figure 6-9
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6 RMO Subsystems and Use Cases/Activities from Event Table Systems Analysis and Design in a Changing World, 5th Edition19 Figure 6-10
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6 Context Diagram for RMO Order-Entry Subsystem Systems Analysis and Design in a Changing World, 5th Edition20 Figure 6-11
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6 Five Separate DFD Fragments for RMO Order-Entry Subsystem Systems Analysis and Design in a Changing World, 5th Edition21 Figure 6-12
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6 Decomposing DFD Fragments Most DFD fragments can be further described using structured English Sometimes DFD fragments need to be diagrammed in more detail Decomposed into subprocesses in a detailed DFD DFD numbering scheme Hierarchical decomposition DFD Fragment 2 is decomposed into Diagram 2 Diagram 2 has processes 2.1, 2.2, 2.3, 2.4 Systems Analysis and Design in a Changing World, 5th Edition22
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6 Detailed DFD for Create new order DFD Fragment Systems Analysis and Design in a Changing World, 5th Edition23 Figure 6-14
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6 Physical and Logical DFDs Logical model Assumes implementation in perfect technology Does not tell how system is implemented Physical model Describes assumptions about implementation technology Developed in last stages of analysis or in early design Systems Analysis and Design in a Changing World, 5th Edition24
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6 Physical DFD for Scheduling Courses Systems Analysis and Design in a Changing World, 5th Edition25 Figure 6-15
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6 Evaluating DFD Quality Readable Internally consistent and balanced Accurately represents system requirements Reduces information overload – rule of 7 +/- 2 Single DFD should not have more than 7 +/-2 processes No more than 7 +/- 2 data flows should enter or leave a process or data store in a single DFD Minimizes required number of interfaces Systems Analysis and Design in a Changing World, 5th Edition26
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6 Data Flow Consistency Problems Differences in data flow content between a process and its process decomposition Data outflows without corresponding inflows Data inflows without corresponding outflows Results in unbalanced DFDs Systems Analysis and Design in a Changing World, 5th Edition27
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6 Consistency Rules All data that flows into a process must Flow out of the process, or Be used to generate data that flows out of the process All data that flows out of a process must Have flowed into the process, or Have been generated from data that flowed into the process Systems Analysis and Design in a Changing World, 5th Edition28
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6 Unnecessary Data Input: Black Hole Systems Analysis and Design in a Changing World, 5th Edition29 Figure 6-16
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6 Process with Impossible Data Output: a Miracle معجزة Systems Analysis and Design in a Changing World, 5th Edition30 Figure 6-17
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6 Process with Unnecessary Data Input Systems Analysis and Design in a Changing World, 5th Edition31 Figure 6-18
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6 Process with Impossible Data Output Systems Analysis and Design in a Changing World, 5th Edition32 Figure 6-19
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6 Documentation of DFD Components Lowest-level processes need to be described in detail Data flow contents need to be described Data stores need to be described in terms of data elements Each data element needs to be described Various options for process definition exist Systems Analysis and Design in a Changing World, 5th Edition33
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6 Structured English Method of writing process specifications Combines structured programming techniques with narrative English Well-suited for lengthy sequential processes or simple control logic (single loop or if-then-else) Ill-suited for complex decision logic or few (or no) sequential processing steps Systems Analysis and Design in a Changing World, 5th Edition34
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6 Structured English Example Systems Analysis and Design in a Changing World, 5th Edition35 Figure 6-20
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6 Process 2.1 and Structured English Process Description Systems Analysis and Design in a Changing World, 5th Edition36 Figure 6-21
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6 Decision Tables and Decision Trees Can summarize complex decision logic better than structured English Incorporate logic into the table or tree structure to make descriptions more readable Systems Analysis and Design in a Changing World, 5th Edition37
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6 Decision Table for Calculating Shipping Charges Systems Analysis and Design in a Changing World, 5th Edition38 Figure 6-23
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6 Decision Tree for Calculating Shipping Charges Systems Analysis and Design in a Changing World, 5th Edition39 Figure 6-24
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6 Data Element Definitions Data type description String, integer, floating point, Boolean Sometimes very specific written description Length of element Maximum and minimum values Data dictionary – repository for definitions of data flows, data stores, and data elements Systems Analysis and Design in a Changing World, 5th Edition40
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6 Data Element Definition Examples Systems Analysis and Design in a Changing World, 5th Edition41 Figure 6-30
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6 Components of a Traditional Analysis Model Systems Analysis and Design in a Changing World, 5th Edition42 Figure 6-31
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6 Summary Data flow diagrams (DFDs) are used in combination with event table and entity-relationship diagram (ERD) to model system requirements DFDs model system as set of processes, data flows, external agents, and data stores DFDs easy to read – graphically represent key features of system using small set of symbols Many types of DFDs – context diagrams, DFD fragments, subsystem DFDs, event-partitioned DFDs, and detailed process DFDs Systems Analysis and Design in a Changing World, 5th Edition43
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6 Summary (continued) Each process, data flow, and data store requires detailed definition Analyst may define processes as structured English process specifications, decision tables, decision trees, or detail process DFDs Detailed process decomposition DFDs used when internal process complexity is great Data flows are defined by component data elements and their internal structure (algebraic notation) Systems Analysis and Design in a Changing World, 5th Edition44
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